Has anyone heard the usual pearl of wisdom "it's to cold to snow" yet this year? People who say that should left out on the frozen moors in just their undercrackers.
"Too cold to snow" is, as you say Not Quite Right. But it's an understandable one because as it gets really cold, you often experience much
less snow. For instance, apparently Nashville
* gets more snow in any given year than the south pole. So it might be a little cruel to shove a chap out in the cold in nothing more than his underthings just for having a poor grasp of meteorological physics...
It's all to do with moisture levels. To be precise; water vapour. Snow starts to form in the atmosphere as water condenses into a tiny droplet. As more and more water vapor condenses onto its surface, the droplet grows. Cold air then freezes this water into an ice crystal. The coldest weather is almost always associated with very high pressure and very dry air. If there ain't much moisture, there can't be much snow.
At really chilly temperatures, the saturation vapor pressure is much lower than in slightly warmer areas. The 'Dew-point' is defined as the temperature at which the air, when cooled, will
just become saturated. Or in other words, the relative humidity must rise to at least 100%.
At this point, the water vapour ought to start to condense out as a liquid if the dew point is above freezing. If the dew point is below freezing, it'll become tiny ice crystals.
But this doesn't always happen, and the relative humidity can actually exceed 100% (also known as supersaturation). The vapor can usually change phase only if there is something for it to cling onto. Believe it or not, even up in the clouds there are enough tiny particles called nuclei, that do just this. Neat, huh?
But, I hear you saying, "That's all very well but what are you ranting on about, woman! Anyways, there's a whole shed load of snow at the South Pole!"
Yeah, but in Nashville, it
melts every year, innit?
*Like many of us, I was raised in Middle England, not in Nashville Tennessee...
